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Dynamic JSON Function

I am using a getJSON method to post the data I have in a database, through a for loop and into an HTML page. But I would like to the function to call different tables in my database depending on the integer the for loop is currently on, something like this:
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?", function(table+r) {
//function stuff here
});
}
But when I try to do this, the "table+r" is flagging a syntax error. What am I doing wrong?

You are defining a function, not calling it. Between ( and ) you have to put identifiers (variable names) not expressions.
To pass data here, you need to use variables from a wider scope than the function. Since the variable is going to change (and the function is called asynchronously) you have to use a closure to do this.
function mkCallback(table) {
var foo = "table" + table;
return function () {
// function stuff that uses foo here
// foo from the time mkCallback was called to make this function
// will still be in scope
};
}
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?", mkCallback(table+r));
}

function(table+r) { tries to create a function with table+r as a parameter, but + is not valid in a variable name. I think you instead want something like this:
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?",
(function(currentR){
return function() {
var someVariable=table+currentR; // No idea where table came from...
//function stuff here
}
})(r));
}
As #Quentin mentioned by the time the callback is called, r will have reached its final value, hence the interesting closure.

I think what you probably want is
for (var r = 0; r < 8; r++){ //outer loop
function(tablenum){ //closure function
tablename = table+tablenum // saved reference to "table+r"
$.getJSON("PHP-PAGE.php?jsoncallback=?", function() {
//function stuff here, using tablename as the param
});
}(r)
}
This creates a closure to maintain the value of the iterated value. You can reference tablename in the callback function, and that will refer to a value equivalent to table+r
The issues with your original example
You were putting table+r as a parameter to a function you were defining, rather than an argument to one you were calling
You were trying to get the callback to reference r. But the callback won't run until after the loop has executed, so r will be 8 for all callback functions.
If you were trying to reference "table1", "table2" then you want to have "table"+r. Otherwise I assume you're referencing a table variable outside the scope of the code you showed us.

You can directly reference the variable r in your callback. Not sure what table is - the return data from the JSON call? Try the following:
for (var r = 0; r < 8; r++){
$.getJSON("PHP-PAGE.php?jsoncallback=?", function(jsonReturnData) {
//function stuff here
alert(r);
});
}

Why are objects' values captured inside function calls?

This code is supposed to pop up an alert with the number of the image when you click it:
for(var i=0; i<10; i++) {
$("#img" + i).click(
function () { alert(i); }
);
}
You can see it not working at http://jsfiddle.net/upFaJ/. I know that this is because all of the click-handler closures are referring to the same object i, so every single handler pops up "10" when it's triggered.
However, when I do this, it works fine:
for(var i=0; i<10; i++) {
(function (i2) {
$("#img" + i2).click(
function () { alert(i2); }
);
})(i);
}
You can see it working at http://jsfiddle.net/v4sSD/.
Why does it work? There's still only one i object in memory, right? Objects are always passed by reference, not copied, so the self-executing function call should make no difference. The output of the two code snippets should be identical. So why is the i object being copied 10 times? Why does it work?
I think it's interesting that this version doesn't work:
for(var i=0; i<10; i++) {
(function () {
$("#img" + i).click(
function () { alert(i); }
);
})();
}
It seems that the passing of the object as a function parameter makes all the difference.
EDIT: OK, so the previous example can be explained by primitives (i) being passed by value to the function call. But what about this example, which uses real objects?
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
toggler.click(function () { toggler.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(toggler);
}
Not working: http://jsfiddle.net/Zpwku/
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
(function (t) {
t.click(function () { t.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(t);
})(toggler);
}
Working: http://jsfiddle.net/YLSn6/

Most of the answers are correct in that passing an object as a function parameter breaks a closure and thus allow us to assign things to functions from within a loop. But I'd like to point out why this is the case, and it's not just a special case for closures.
You see, the way javascript passes parameters to functions is a bit different form other languages. Firstly, it seems to have two ways of doing it depending on weather it's a primitive value or an object. For primitive values it seems to pass by value and for objects it seems to pass by reference.
How javascript passes function arguments
Actually, the real explanation of what javascript does explains both situations, as well as why it breaks closures, using just a single mechanism.
What javascript does is actually it passes parameters by copy of reference. That is to say, it creates another reference to the parameter and passes that new reference into the function.
Pass by value?
Assume that all variables in javascript are references. In other languages, when we say a variable is a reference, we expect it to behave like this:
var i = 1;
function increment (n) { n = n+1 };
increment(i); // we would expect i to be 2 if i is a reference
But in javascript, it's not the case:
console.log(i); // i is still 1
That's a classic pass by value isn't it?
Pass by reference?
But wait, for objects it's a different story:
var o = {a:1,b:2}
function foo (x) {
x.c = 3;
}
foo(o);
If parameters were passed by value we'd expect the o object to be unchanged but:
console.log(o); // outputs {a:1,b:2,c:3}
That's classic pass by reference there. So we have two behaviors depending on weather we're passing a primitive type or an object.
Wait, what?
But wait a second, check this out:
var o = {a:1,b:2,c:3}
function bar (x) {
x = {a:2,b:4,c:6}
}
bar(o);
Now see what happens:
console.log(o); // outputs {a:1,b:2,c:3}
What! That's not passing by reference! The values are unchanged!
Which is why I call it pass by copy of reference. If we think about it this way, everything makes sense. We don't need to think of primitives as having special behavior when passed into a function because objects behave the same way. If we try to modify the object the variable points to then it works like pass by reference but if we try to modify the reference itself then it works like pass by value.
This also explains why closures are broken by passing a variable as a function parameter. Because the function call will create another reference that is not bound by the closure like the original variable.
Epilogue: I lied
One more thing before we end this. I said before that this unifies the behavior of primitive types and objects. Actually no, primitive types are still different:
var i = 1;
function bat (n) { n.hello = 'world' };
bat(i);
console.log(i.hello); // undefined, i is unchanged
I give up. There's no making sense of this. It's just the way it is.

It's because you are calling a function, passing it a value.
for (var i = 0; i < 10; i++) {
alert(i);
}
You expect this to alert different values, right? Because you are passing the current value of i to alert.
function attachClick(val) {
$("#img" + val).click(
function () { alert(val); }
);
}
With this function, you'd expect it to alert whatever val was passed into it, right? That also works when calling it in a loop:
for (var i = 0; i < 10; i++) {
attachClick(i);
}
This:
for (var i = 0; i < 10; i++) {
(function (val) {
$("#img" + val).click(
function () { alert(val); }
);
})(i);
}
is just an inline declaration of the above. You are declaring an anonymous function with the same characteristics as attachClick above and you call it immediately. The act of passing a value through a function parameter breaks any references to the i variable.

upvoted deceze's answer, but thought I'd try a simpler explanation. The reason the closure works is that variables in javascript are function scoped. The closure creates a new scope, and by passing the value of i in as a parameter, you are defining a local variable i in the new scope. without the closure, all of the click handlers you define are in the same scope, using the same i. the reason that your last code snippet doesn't work is because there is no local i, so all click handlers are looking to the nearest parent context with i defined.
I think the other thing that might be confusing you is this comment
Objects are always passed by reference, not copied, so the self-executing function call should make no difference.
this is true for objects, but not primitive values (numbers, for example). This is why a new local i can be defined. To demonstrate, if you did something weird like wrapping the value of i in an array, the closure would not work, because arrays are passed by reference.
// doesn't work
for(var i=[0]; i[0]<10; i[0]++) {
(function (i2) {
$("#img" + i2[0]).click(
function () { alert(i2[0]); }
);
})(i);
}

In the first example, there is only one value of i and it's the one used in the for loop. This, all event handlers will show the value of i when the for loop ends, not the desired value.
In the second example, the value of i at the time the event handler is installed is copied to the i2 function argument and there is a separate copy of that for each invocation of the function and thus for each event handler.
So, this:
(function (i2) {
$("#img" + i2).click(
function () { alert(i2); }
);
})(i);
Creates a new variable i2 that has it's own value for each separate invocation of the function. Because of closures in javascript, each separate copy of i2 is preserved for each separate event handler - thus solving your problem.
In the third example, no new copy of i is made (they all refer to the same i from the for loop) so it works the same as the first example.

Code 1 and Code 3 didn't work because i is a variable and values are changed in each loop. At the end of loop 10 will be assigned to i.
For more clear, take a look at this example,
for(var i=0; i<10; i++) {
}
alert(i)
http://jsfiddle.net/muthkum/t4Ur5/
You can see I put a alert after the loop and it will show show alert box with value 10.
This is what happening to Code 1 and Code 3.

Run the next example:
for(var i=0; i<10; i++) {
$("#img" + i).click(
function () { alert(i); }
);
}
i++;
You'll see that now, 11 is being alerted.
Therefore, you need to avoid the reference to i, by sending it as a function parameter, by it's value. You have already found the solution.

One thing that the other answers didn't mention is why this example that I gave in the question doesn't work:
for(var i=0; i<5; i++) {
var toggler = $("<img/>", { "src": "http://www.famfamfam.com/lab/icons/silk/icons/cross.png" });
toggler.click(function () { toggler.attr("src", "http://www.famfamfam.com/lab/icons/silk/icons/tick.png"); });
$("#container").append(toggler);
}
Coming back to the question months later with a better understanding of JavaScript, the reason it doesn't work can be understood as follows:
The var toggler declaration is hoisted to the top of the function call. All references to toggler are to the same actual identifier.
The closure referenced in the anonymous function is the same (not a shallow copy) of the one containing toggler, which is being updated for each iteration of the loop.
#2 is quite surprising. This alerts "5" for example:
var o;
setTimeout(function () { o = {value: 5}; }, 100);
setTimeout(function () { alert(o.value) }, 1000);

Wrong Parameter is passed to function

I have the following code that adds an onmouseover event to a bullet onload
for (var i = 0; i <= 3; i++) {
document.getElementById('menu').getElementsByTagName('li')[i].onmouseover = function () { addBarOnHover(i); };
}
This is the function that it is calling. It is supposed to add a css class to the menu item as the mouse goes over it.
function addBarOnHover(node) {
document.getElementById('menu').getElementsByTagName('li')[node].className = "current_page_item"; }
When the function is called, I keep getting the error:
"document.getElementById("menu").getElementsByTagName("li")[node] is
undefined"
The thing that is stumping me is I added an alert(node) statement to the addBarOnHover function to see what the value of the parameter was. The alert said the value of the parameter being passed was 4. I'm not sure how this could happen with the loop I have set up.
Any help would be much appreciated.

This is a common problem when you close over an iteration variable. Wrap the for body in an extra method to capture the value of the iteration variable:
for (var i = 0; i <= 3; i++) {
(function(i){ //here
document.getElementById('menu').getElementsByTagName('li')[i].onmouseover = function () { addBarOnHover(i); };
})(i); //here
}
an anonymous function is created each time the loop is entered, and it is passed the current value of the iteration variable. i inside the anonymous function refers to the argument of this function, rather than the i in the outer scope.
You could also rename the inner variable for clarity:
for(var i=0; i<=3; i++){
(function(ii){
//use ii as i
})(i)
}
Without capturing the iteration variable, the value of i when it is finally used in the anonymous handler has been already changed to 4. There's only one i in the outer scope, shared between all instances of the handler. If you capture the value by an anonymous function, then the argument to that function is used instead.

i is being passed as a reference (not by value), so once the onmouseover callback is called, the value of i has already become 4.
You'll have to create your callback function using another function:
var menu = document.getElementById('menu');
var items = menu.getElementsByTagName('li');
for (var i = 0; i <= 3; i++) {
items[i].onmouseover = (function(i) {
return function() {
addBarOnHover(i);
};
})(i);
}
You could make it a little more readable by making a helper function:
var createCallback = function(i) {
return function() {
addBarOnHover(i);
};
};
for (var i = 0; i <= 3; i++) {
items[i].onmouseover = createCallback(i);
}

Javascript,Setting up onclick method syntax

I am looking at a javascript code that manipulates an HTML A tag , and I'm having trouble understanding how it sets up the "onclick" property. It seems to be telling it to update ytplayer_playitem with the index variable j and then call ytplayer_playlazy(1000)
But what's up with all the parentheses? What details in the javascript syntax allows it to be setup like this?
var a = document.createElement("a");
a.href = "#ytplayer";
a.onclick = (function (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);

Well, basically, the value of onclick is a function that will get called when the element is clicked. Whatever you want to happen when the user clicks the element goes in the body of the function.
You could create a named function and then assign it to the element's onclick attribute:
function youClickedMe() {
...
}
a.onclick = youClickedMe
but that clutters up the namespace with a function name that is never referenced anywhere else. It's cleaner to create an anonymous function right where you need it. Normally, that would look like this:
a.onclick = function() { ... }
But if we try that with your specific example:
a.onclick = function() {
ytplayer_playitem = something; // ??
ytplayer_playlazy(1000);
}
We see that it hard-codes the something that gets played. I'm assuming the original code was taken from a loop which generates several clickable links to play; with the code just above, all of those links would play the same thing, which is probably not what you want.
So far, so straightforward, but this next leap is where it gets tricky. The solution seems obvious: if you're in a loop, why not just use the loop variable inside the function body?
// THIS DOESN'T WORK
a.onclick = function() {
ytplayer_playitem = i;
ytplayer_playlazy(1000);
}
That looks like it should work, but unfortunately the i inside the function refers to the value of the variable i when the function is called, not when it's created. By the time the user clicks on the link, the loop that created all the links will be done and i will have its final value - probably either the last item in the list or one greater than that item's index, depending on how the loop is written. Whatever its value is, you once again have the situation where all links play the same item.
The solution in your code gets a little meta, by using a function whose return value is another function. If you pass the loop control variable to the generating function as an argument, the new function it creates can reference that parameter and always get the value that was originally passed in, no matter what has happened to the value of the outer argument variable since:
function generate_onclick(j) {
// no matter when the returned function is called, its "j" will be
// the value passed into this call of "generate_onclick"
return function() { ytplayer_playitem = j; ytplayer_playlazy(1000); }
}
To use that, call it inside the loop like this:
a.onclick = generate_onclick(i);
Each generated function gets its very own j variable, which keeps its value forever instead of changing when i does. So each link plays the right thing; mission accomplished!
That's exactly what your posted original code is doing, with one small difference: just like the first step in my explanation, the author chose to use an anonymous function instead of defining a named one. The other difference here is that they are also calling that anonymous function immediately after defining it. This code:
a.onclick = (function (j) { ... })(i)
is the anonymous version of this code:
function gen(j) { ... }
a.onclick = gen(i)
The extra parens around the anonymous version are needed because of JavaScript's semicolon-insertion rules; function (y) {...}(blah) compiles as a standalone function definition followed by a standalone expression in parentheses, rather than a function call.

"But what's up with all the parentheses? "
Most of the parentheses are just doing what you'd expect.
There's an extra set that isn't technically needed, but is often used as a hint that the function is being invoked.
// v-v---these are part of the function definition like normal
a.onclick = (function (j) {
// ^-----------this and...v
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
// v---...this are technically not needed here, but are used as a hint
})(i);
// ^-^---these invoked the function like normal
"What details in the javascript syntax allows it to be setup like this?"
The upshot is that the function is invoked immediately, and passed i so that its value is referenced by the j parameter in the immediately invoked function.
This creates a variable scope that the returned function will continue to have access to. This way it always has access to the j variable, and not the i that gets overwritten in the loop.
These inlined functions are abused a bit IMO. It becomes clearer if you simply make it a named function.
for(var i = 0; i < 10; i++) {
// create the new element
a.onclick = createHandler(i);
// append it somewhere
}
function createHandler (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
}
The resulting handler is exactly the same, but the code is much less cryptic.

Right, I'm going to guess that the surrounding code looks like this:
for (var i = 0; i < playitems.length; i++) {
// above code here
}
Now, you could do the obvious thing here, and assign the onclick property like this:
a.onclick = function() {
ytplayer_playitem = i;
ytplayer_playlazy(1000);
};
However that wouldn't work very well, because the value of i changes. Whichever link was clicked, the last one would be the one activated, because the value of i at that point would be the last one in the list.
So you need to prevent this happening. You need to do this by creating a new scope, which is done by creating an extra function, which is immediately invoked:
(function (j) {
// some code here
})(i);
Because i has been passed into the function, the value is passed rather than a reference to the variable being kept. This means that you can now define a function which will have a reference to the correct value. So you get your extra function to return the click handling function:
a.onclick = (function (j) { // j is the right number and always will be
return function () { // this function is the click handler
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
So each a element has its own click handler function, each of which has its own individual j variable, which is the correct number. So the links, when clicked, will perform the function you want them to.

a.onclick = (function (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
This creates a "closure" to ensure that the value of i that is bound to the handler is the value of i "at that time" and not i in the general.
In your code, the function inside the () is an expression, executed and passed the variable i. This is the (i) you see in the end part. In this executed function expression, the i becomes the local variable j. This executed function expression returns the handler function that is to be bound the onclick event carrying the value of j which was i "at that time"
if i did not use the closure:
//suppose i is 1
var i = 1;
a.onclick = function () {
ytplayer_playitem = i;
ytplayer_playlazy(1000);
};
//and changed i
i = 2;
//if you clicked the <a>, it would not be 1 onclick but 2 because you
//did not assign the value of i "at that time". i is "tangible" this way

a.onclick = (function (j) {
return function () {
ytplayer_playitem = j;
ytplayer_playlazy(1000);
};
})(i);
What you have here is a self-invoking anonymous function. Let's break it down, first replacing the body of the function with something simpler (return j + 1;):
function( j ) { return j + 1; }
This s a run-of-the-mill anonymous function or closure. This line of code is an expression, and so it has a value, and that value is a function. Now we could do this:
var foo = function( j ) { return j + 1; }
foo( 5 ); // => 6
You recognize this, I'm sure—we're assigning the anonymous function to the variable foo, and then calling the function by name with the argument i. But, instead of creating a new variable, because the closure is an expression we can call it like this instead:
( function( j ) { return j + 1; } )( 5 ); // => 6
Same result. Now, it's just returning j + 1 but in your code it returns something else: Another anonymous function:
return function() { /* ... */ }
What happens when we have a self-invoking anonymous function that returns a function? The result is the "inner" function that was returned:
a.onclick = ( function( j ) {
return function() {
ytplayer_playitem = j;
ytplayer_playlazy( 1000 );
}
}
)( i );
If i was equal to 9 then a.onclick would now hold a function equivalent to this:
function() {
ytplayer_playitem = 9;
ytplayer_playlazy( 1000 );
}
As others have pointed out, the usefulness of this is that when ( function( j ) { /* ... */ } )( i ) is invoked you are capturing the value of i at that time and putting it into j rather than creating a reference to the value i holds, which may (and probably will) change later on.

Javascript scope and calling a function

My code:
for (var i = 0; i < mapInfos.length; i++) {
var x = function () { doStuff(i); };
google.maps.event.addListenerOnce(mapInfos[i].map, 'tilesloaded', x);
}
The doStuff method simply alerts the value of i. mapInfos has two entries, so you'd expect it to alert 0 and 1, but instead it alerts 2 and 2. I can appreciate vaguely why it is doing this (although var i should keep it local to the scope of the loop?) but how can I make it work as intended?

edit — note that when first posted, the original question included a link to a jsfiddle that seemed to be a relevant example of what the current question is trying to achieve, only it appears to work ...
The code in the jsfiddle works because there's only one "i" in that code. The "i" used in the second loop (where the functions are actually called) is the same "i" as used in the first loop. Thus, you get the right answer because that second loop is running "i" through all the values from zero through four again. If you added:
i = 100;
functions[0]();
you'd get 100 printed out.
The only way to introduce a new scope in JavaScript is a function. One approach is to write a separate "function maker" function:
function makeCallback(param) {
return function() {
doStuff(param);
};
}
Then in your loop:
for (var i = 0; i < mapInfos.length; i++) {
var x = makeCallback(i);
google.maps.event.addListenerOnce(mapInfos[i].map, 'titlesloaded', x);
}
That'll work because the call to the "makeCallback" function isolates a copy of the value of "i" into a new, unique instance of "param" in the closure returned.

Create a new scope for it.
Functions create scope.
function doStuffFactory(i) {
return function () { doStuff(i); };
}
for (var i = 0; i < mapInfos.length; i++) {
var x = doStuffFactory(i);
google.maps.event.addListenerOnce(mapInfos[i].map, 'tilesloaded', x);
}

Change it to
var x = function (param) { doStuff(param); };
Obviously what is going on is that you are alerting a variable that is changing. With the above change it copies it so even if i changes it will still alert the right value.

Javascript doesn't have block scope, so you don't get an x that's local to the loop. Yea!
It has function scope, though.

Yep, weird isn't it!Pointy has an explanation
I have no idea why your first example worked (I wasn't expecting it to) Pointy has an explanation of why your first example worked - The reason why your second one doesn't is because i is scoped to the function containing the for loop, not to the scope defined by the for loop. In fact the only things that have scope in JavaScript are functions. This means that by the time your function gets executed i is 2.
What you need to do is create a scope, for example:
for (var i = 0; i < mapInfos.length; i++) {
var x = (function() {
return function () { doStuff(i); };
})(i);
google.maps.event.addListenerOnce(mapInfos[i].map, 'tilesloaded', x);
}
See JavaScript closures in for-loops for more.